Seminar: 3-D Borehole Seismic Techniques – Application to Hardrock Characterisation and Mineral Exploration

by Dr. Calin Cosma, Owner, Vibrometric, Helsinki, Finland

Monday 15 April 2019, 11AM – 12PM, ARRC Auditorium

ABSTRACT: Newly discovered superior-value mineral deposits are increasingly being hosted in deep geological features with intricate shapes and diverse physical properties. Mineralised zones and rock structures of interest to mining produce distinguishable responses when probed with seismic waves and deep targets with complex shapes can be imaged directly from their immediate proximity by borehole seismic techniques. Seismic measurements with sources and receivers placed in boreholes offer a unique combination of investigation depth of one kilometer or more and a level of detail of one metre or less.

The increased use of polarised sources and multi-component receivers augment the data dimensional complexity over the three spatial dimensions and time. Innovative machine learning technology can help extract meaningful data attributes from noise. Measuring tools and procedures used in other fields can produce higher resolution results and lead to a better understanding of the mineral deposits. Storage and disposal facilities built deep in the bedrock provide opportunities to characterise the rock at various scales, which can be readily applied to hard rock exploration and mining. Notable examples are given from rock characterisation projects focused on deformation zones and large fractures at Olkiluoto, Finland and Äspö, Sweden. Whenever possible, there is an advantage in performing 3D borehole seismic imaging surveys at the same time as acquiring surface data. A better coverage in depth over a wider area is thus achieved.

Surface 3D and 3D VSP were applied jointly within pre-feasibility studies for mine development at various locations, including the Kevitsa mine in Northern Finland and the Millennium uranium deposit, in the Athabasca basin of northern Saskatchewan, Canada. These sites are representative by the diversity of the geological conditions and the different challenges they present to any investigation method. In particular, seismic methods, and among these, borehole techniques, were identified as the best tools to potentially characterise alteration and structurally compromised zones. In addition to 3D surface and 3D VSP, significantly more resolved – albeit also more local – images are obtained by downhole and cross-hole side-scan seismic surveys, with sources and receives placed down hole. Such surveys were conducted in Canada at Voisey’s Bay already nearly two decades ago and more recently, with updated technology, at the McArthur River and Snap Lake mines. In-mine 3D borehole seismic imaging techniques can provide continuity information for targets detected locally in galleries and in boreholes.

Reflection seismic imaging from boreholes of a massive sulfide ore deposit was done in the Kylylahti Cu-Au-Zn mine in eastern Finland. The measurements were done by three-component geophones in four boreholes and a fiber-optic Distributed Acoustic Sensing (DAS) cable in one borehole. Both 3-component geophone and DAS VSP methodologies correctly imaged the sub-vertical ore deposit. However, the processing and interpretation of the seismic records was not straightforward due to the geological complexity of the area and elastic three-dimensional full-waveform seismic modeling was conducted to understand the understand the data and represent the results with respect to the geological origins. While focusing on deep targets and producing more detailed and more reliable results in shorter time, recent developments of the borehole seismic methodology strive to minimise cumbersome instrumental deployment at surface and bring the cost of seismic to levels comparable or lower than the cost of drilling.

BIOGRAPHY: Calin Cosma has a broad background in the development of specialised geophysical tools, geophysical imaging and rock characterisation. He holds a PhD in physics and provided notable contributions to the development of detailed scale and deep seismic imaging methods and techniques.

Areas of expertise include: site characterisation (prediction of weak zones and faults, rock quality assessment); mining (ore prospecting and delineation, mapping of ore bodies, dykes, seams and structure); oil (fractured reservoir characterisation, 3D fracture mapping from VSP data); tunnels (prediction of rock anomalies ahead of works and around galleries) & location of old shafts, drifts, caverns; nuclear and hydroelectric power plants (foundation and underground facilities); ground engineering & control of man-made structures; dams and dam sites (detection of leakage, mapping of karsts, checking of grout injection, etc.); bridges (detection of fissures, sand pockets and unconsolidated concrete); chemical and radioactive waste; water resources; environmental contamination; storage for oil, gas, hot water.

Achievements of possible interest: Development of novel seismic tools, including specialized sources, data acquisition and handling techniques for rockmass characterisation and monitoring.